2.4.2[01] i386 does not reenable local apic

Initial /proc/cpuinfo (Compaq Evo N800v).

processor : 0
vendor_id : GenuineIntel
cpu family : 15
model : 2
model name : Mobile Intel(R) Pentium(R) 4 - M CPU 2.00GHz
stepping : 7
cpu MHz : 1993.585
cache size : 512 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts mmx fxsr sse sse2 ss ht tm
bogomips : 3971.48

arc/i386/kernel/apic.c::detect_init_APIC()

	switch (boot_cpu_data.x86_vendor) {
	case X86_VENDOR_AMD:
		if (boot_cpu_data.x86 == 6 && boot_cpu_data.x86_model > 1)
			break;
		if (boot_cpu_data.x86 == 15 && cpu_has_apic)
			break;
		goto no_apic;
	case X86_VENDOR_INTEL:
		if (boot_cpu_data.x86 == 6 ||
		    (boot_cpu_data.x86 == 15 && cpu_has_apic) ||
		    (boot_cpu_data.x86 == 5 && cpu_has_apic))
			break;
		goto no_apic;
	default:
		goto no_apic;
	}

	if (!cpu_has_apic) {
		/*
		 * Some BIOSes disable the local APIC in the
		 * APIC_BASE MSR. This can only be done in
		 * software for Intel P6 and AMD K7 (Model > 1).
		 */
		rdmsr(MSR_IA32_APICBASE, l, h);
		if (!(l & MSR_IA32_APICBASE_ENABLE)) {
			printk("Local APIC disabled by BIOS -- reenabling.\n");
			l &= ~MSR_IA32_APICBASE_BASE;
			l |= MSR_IA32_APICBASE_ENABLE | APIC_DEFAULT_PHYS_BASE;
			wrmsr(MSR_IA32_APICBASE, l, h);
		}
	}

Because boot_cpu_data.x86 == 15 and the local APIC is disabled in BIOS, detect_init_APIC() skips the code that reenables the local APIC. This test is back to front. Change the code to force X86_VENDOR_INTEL to drop through and it successfully reenables the local apic, I even get NMI events.

The correct fix is to delete the "&& cpu_has_apic" test for family 15 CPUs. Simply falling through for VENDOR_INTEL will oops UP P5s.

I've seen a few P4 owners being caught by this. OTOH, several of those had crap BIOSen that caused hangs with local APIC enabled.

I'll do a patch for 2.4.22-pre, if we ever actually get a 2.4.21 final...

/Mikael